In printed circuit board manufacture, the surface or selected areas of the surface of an insulating base are treated to render them receptive to the deposition of metal from an electroless plating solution. In one type of procedure, portions of the surface of the insulating base are sensitized in the form of a desired printed circuit pattern and copper is deposited on the sensitized areas, as by electroless means alone or in combination with electroplating, to form the desired pattern of conductor lines. In another type of procedure, a layer of an adhesion promoting adhesive is interposed between the insulating base and the metal deposit.
In the typical case, through holes are provided in the insulating base, as by drilling or punching, for purposes of forming interconnections and the hole walls are sensitized for metal deposition. On the other hand, the insulating base can be made in whole or in part of a material already catalytic to the electroless deposition of metal and when the holes are formed the catalytic material becomes exposed, thus dispensing with the need for sensitization.
Because the through hole plating and insulating substratum have different coefficients of expansion, the hole wall metal expands or contracts at a different rate than the substratum when the board is exposed to unusually high or low temperature conditions varying from the ambient. This sometimes results in the development of cracks or fissues in the hole wall plating leading to breaks in the conducting pathways and failures during use. The metal deposit is also characterized by an internal stress, which contributes toward crack formation. Metals of relatively low ductility especially are susceptible to cracking when highly stressed.
This problem of crack formation is encountered during high temperature soldering operations in printed circuit board manufacture. In one such procedure, solder is applied, as by floating on a solder bath or by passage over a solder wave, to each of the through holes to which circuit components such as integrated circuits, resistors, capacitors, and the like are later attached. The solder also helps to prevent the metallized hole walls from corroding which can occcur if the circuit boards are stored for periods of time before attachment of the aforementioned circuit components.
In another such soldering operation, after imposing the circuit pattern and coating the panel except the through holes with a mask comprising a solder resist, the board is immersed in a solder bath and solder is permitted to substantially fill the holes, and while the solder is still molten the excess is blown out of the holes leaving a thin layer of solder coating the hole walls.
In subsequent stages of manufacture, molten solder is applied to substantially fill the remainder of the holes contaning component lead and thus affix the components to the board.
These solder treatments, even though brief, often induce heat shock and cause cracks to form in the through hole platings rendering the boards susceptible to failure as previously explained.